Safer cars, costly repairs

 

As motor vehicle manufacturers develop and implement new technology to make cars more pedestrian friendly, UK research body Thatcham takes a look at this new electronic gadgetry and how it is affecting the cost of repairs.

Although vehicles are becoming safer and road deaths continue to decline, vulnerable road users account for a high proportion of those who are killed and seriously injured every year. Pedestrians now account for 22 per cent of reported deaths and serious injuries on UK roads.

In 2009, the number of children aged under 15 killed or seriously injured (KSI) was 2671; and the majority of child KSI casualties are pedestrians, accounting for 62 per cent of the total in 2009.

A car-to-pedestrian collision involves complex dynamics as the car strikes the pedestrian, and this can be separated into several stages.

First stage

The car bumper contacts the pedestrian’s leg – the bumper strike. The bumper strike can result in lower leg injuries, and knee injuries that are particularly debilitating and costly.

Second stage

The bonnet leading edge strike, where the pedestrian’s upper leg is struck by the top of the bumper and the front edge of the bonnet, causing severe pelvic injuries.


Third stage

The pedestrian rolls over the front of the car and onto the bonnet – the bonnet strike.
The pedestrian’s upper body and head will strike onto the bonnet, which is usually made from sheet metal and can provide some energy absorption.
The bonnet is crushed downward by the pedestrian until it contacts with the engine and stiff parts underneath.

The engine, suspension turrets, and windscreen wiper spindles are the parts that most often cause injury to pedestrians.

Depending upon the speed of the impact, pedestrians can often hit the windscreen which is relatively soft, or they will hit the windscreen surround, often very stiff and unforgiving.


Fourth stage

The road strike, which happens as the pedestrian falls to the ground. The pedestrian may fall in many directions, including being pushed forward to land in front of the car, or being thrown up in the air over the top of the car to land behind it. Up to 45 per cent of all pedestrian casualties are reported to be due to the injuries resulting from the road strike following the impact with the car.


Pedestrian friendly

Pedestrian collisions are very complex and depend upon many factors including the speed and trajectory of the pedestrian in relationship to the car, as well as the size, weight and age of the pedestrian.

The design of the vehicle that hits a pedestrian also plays a significant part in determining injury risk.

For example, a low-speed impact might only include bumper contact, whereas a higher speed impact might involve interaction with the bumper, bonnet leading edge, and the bonnet, before throwing pedestrians violently over the top of the car, so they land on the unforgiving road surface.

Research has suggested that car fronts could be more pedestrian friendly, following the pioneering work of Euro NCAP in encouraging best practice pedestrian friendly front ends.
Many manufacturers have introduced pedestrian friendly solutions that can protect the pedestrian from hitting stiff parts of the vehicle by softening the front end of vehicles.

Vehicle design can and does play an increasing role in reducing the risk of pedestrian casualties by the careful design of components that avoid sharp edges and incorporating energy absorbing features into the front end design.


International regulation and consumer testing

Regulatory and consumer organisations carry out tests to assess the level of protection provided by cars in pedestrian collisions. The aim is to save the lives of pedestrians of all shape and sizes by encouraging vehicle manufacturers to introduce features that can protect vulnerable road users.

This also has the positive side effect of negating the emotional trauma suffered by many drivers as they live with the psychological consequences of fatally wounding or injuring a pedestrian.

Although crash tests traditionally use whole dummies to replicate human behaviour, the use of a complete pedestrian surrogate is very difficult due to the complex dynamics of a walking pedestrian and a moving car.

Although it is possible to control the point of impact of the bumper against the pedestrian’s leg, it is difficult to control the precise trajectory of the dummy’s head leading to unrepeatable tests.

To overcome this problem, individual component tests are used. A leg form test assesses the protection afforded to the lower leg by the bumper (bumper strike), an upper leg form assesses the leading edge of the bonnet (bonnet leading edge strike), and child and adult head forms are used to assess the bonnet top area (bonnet strike).

The regulations came into effect in two phases, with the second phase only recently implemented at the end of 2009. The so-called phase two includes modified test parameters and a new time schedule. A new Global Technical Regulation (GTR) on pedestrian protection was also agreed in 2009, which will encourage manufacturers from around the world to also consider pedestrian protection.

Pedestrian protection has also received additional weight within the recently revised Euro NCAP rating scheme.

The new overall rating, which includes pedestrian protection, encourages car makers to improve pedestrian protection, in order to receive 4 or 5-star ratings in the future. In addition, Euro NCAP has recently changed its pedestrian test protocol in order to improve harmonisation with the European regulations, although the speeds and injury criteria are more stringent than the regulation requirements.

Euro NCAP’s tests involve leg form, upper leg form and child/adult head form testing. A series of tests are carried out to replicate accidents involving child and adult pedestrians where impacts occur at 40km/h (25mph).

Impact sites over the front of the car are then assessed and rated as good, adequate, and marginal.

Leg protection can be improved with bumpers which are designed to deform when they hit a pedestrian’s leg.

If the leg is impacted low down, away from the knee, and if the forces are spread over a longer length of leg to increase the area for absorbing energy, then protection is improved.

For the leading edge of the bonnet, improvements can result from the removal of unnecessarily stiff and sharp structures.

The bonnet top area needs to be able to deform and absorb energy in a controlled manner in order to protect the head. However, it is important that sufficient clearance is provided above the stiff engine and suspension structures, which can be contacted by the pedestrian head as the bonnet deforms.


Leg protection

One of Thatcham’s core activities is encouraging better damageability performance in low-speed crashes. In view of this, Thatcham and other Research Council for Automobile Repairs (RCAR) partners have been testing low speed damageability characteristics for many years. Due to issues of bumper height mismatch, underride and suboptimisation, a new test was developed that concentrates on bumper performance alone.

This test, run at 10km/h uses a standardised bumper beam that replicates a real car bumper system.

Thatcham has been publishing bumper test ratings since 2007. The ratings
are generated based on the repair costs and the engagement characteristics of the impact.

Ratings are published for impacts on both the front and rear of the car, and are Good, Acceptable, Marginal or Poor.

For more information on bumper testing see www.thatcham.org/bumpers.

However, some vehicle manufacturers have claimed that good bumper performance to protect the car from damage and control repair costs leads to the design of bumper structures that are incompatible with pedestrian protection.

In the Euro NCAP pedestrian tests, the maximum score for the lower leg impact is six points.

In a collision, the pedestrian’s lower leg typically impacts the bumper region of the vehicle, so it is useful to make a comparison of the bumper ratings against the pedestrian lower leg scores. There is little pattern revealed in this sample of cars; some vehicles with a high lower leg score have a very expensive repair cost.

However, several mainstream manufacturers have scored very highly in the pedestrian lower leg test, but have also managed to control repair costs; reinforcing the view that good bumper performance and high levels of pedestrian protection are not mutually exclusive.

For example, the Toyota Auris has a high lower leg score and is the vehicle with the lowest repair costs, which suggests that pedestrian protection and damageability are not mutually exclusive.

Head protection

One feature of bonnet design that can help to protect pedestrians from severe head injury is to promote the absorption of energy as the head hits the bonnet. Research has shown that bonnets that allow deformation by the head reduce the risk of head injury by up to 30 per cent.

However, head injuries do not typically occur by the head interacting with the bonnet, but with the head riding through the bonnet and hitting the very stiff structure of the engine and suspension towers below.

The position of the bonnet in relation to the engine is largely a matter of styling. Some designs, such as people carriers and small family hatchbacks, tend to have plenty of space beneath the bonnet.

However, some large sports saloons such as BMW, Jaguar and Mercedes tend to favour low bonnets and larger engines, possibly at odds with pedestrian safety. Increasing space between the bonnet and engine during the crash can be a solution and this is where we see the introduction of pedestrian active bonnets.

There are several types of active bonnet system on the market, including systems fitted on the BMW 5-Series, Citroën C6, Honda Legend, Jaguar XF, Mercedes E-Class, and Peugeot RCZ.

Thatcham has assessed three systems, and each one has different features of deployment mechanisms and repair methods.

The Jaguar XF system uses an airbag that is pyrotechnically deployed, which lifts the bonnet upward.

The system on the Mercedes E-Class uses the mechanical deployment of a spring to push the bonnet upward.

The latest BMW 5-Series is similar, since a spring is used to push the bonnet upward, although this is triggered pyrotechnically. Furthermore, the front of the 5-Series bonnet is also pyrotechnically fired upward on springs in the catch.

However, the use of active bonnets raises some questions:

Do the bonnets actually deploy when the car hits a pedestrian?

Do these active bonnets deploy in a typical low-speed impact unnecessarily when there is no pedestrian to protect?

Will active bonnets adversely affect repair costs?

Active bonnet deployments

Thatcham has completed a research project examining a sample of three cars fitted with active bonnets in order to investigate the efficacy of different systems. The aim was to establish whether the bonnets actually deploy when in collision with a pedestrian, but do not deploy when hitting another car.

The tests were also used to measure that amount of time taken for the bonnet to deploy to its fully raised position, as well as to measure the amount of vertical lift.

All three cars, although similar in their general size, construction and target market, have very different pop-up bonnet systems. Both Jaguar and BMW use pyrotechnics and therefore have additional components that are likely to require replacement after of an incorrect deployment.

Mercedes use a mechanical system that is electro-mechanically released.

Since this technology is so new, a test device to measure bonnet triggering is still under development. Two exist today and take the form of legs that are self-standing and are hit by a moving vehicle. One, the Sensor Leg, looks like a human leg and is quite complex, replicating the morphology of an adult leg. The second, known as the PDI leg, represents a child leg, but is visually unlike a human leg.

Thatcham selected three cars from manufacturers that are now fitting these systems. The cars were impacted against these two different leg forms in order to trigger the respective system. The regulatory test protocol requires that active bonnets deploy at speeds between 20km/h and 40km/h, so a test speed of 25km/h was selected.

Repair issues of active bonnets

Any impact with a pedestrian is likely to result in a costly repair bill. The dynamics are complex, and the pedestrian can strike and damage many different parts of the car.

However, another more pressing issue is whether the active bonnet systems themselves are more expensive to repair, for example, if the system were falsely activated in a car-to-car impact (or hitting a wheelie bin) instead of a pedestrian impact. In this case, Thatcham has identified the costs of replacing the necessary parts to bring the system back to working order after deployment.

However, it is important to note that if the system were correctly deployed in a pedestrian impact, there would likely be other costs associated with a pedestrian impact where damage is caused on other parts of the car such as the windscreen, mirrors and roof.
Source: Thatcham Research News Special Edition 10/No.7